AU661132B2

AU661132B2 - Speech signal encoder/decoder device in mobile communication

Info

Publication number: AU661132B2
Application number: AU37024/93A
Authority: AU
Inventors: Yoshihiro Unno
Original assignee: NEC Corp
Current assignee: NEC Corp
Priority date: 1992-04-21
Filing date: 1993-04-20
Publication date: 1995-07-13
Anticipated expiration: 2013-04-20
Also published as: EP0567068B1; DE69322897D1; EP0567068A1; CA2094319C; AU3702493A; CA2094319A1; DE69322897T2

Description

66113 S F Ref: 238488

AUSTRALIA

PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT

ORIGINAL

Name and Address of Applicant: NEC Corporation 7-1, Shiba Minato-ku Tokyo

JAPAN

Actual Inventor(s): Address for Service: Yoshihiro Unno Spruson Ferguson, Patent Attorneys Level 33 St Martins Tower, 31 Market Street Sydney, New South Wales, 2000, Australia Speech Signal Encoder/Decoder Device in Mobile Communication Invention Title: The following statement is a full description of this invention, including the best method of performing it known to me/us:- 5845/3 -1- SPEECH SIGNAL ENCODER/DECODER DEVICE IN MOBILE COMMUNICATION BACKGROUND OF THE INVENTION 1i. Field of the Invention The present invention relates to a speech signal encoder/decoder device in mobile communication, and more specifically to a speech signal encoder/decoder device in mobile communication for use in a cellular automobile telephone system in which transmission lines are inevitably switched during c:t communication.

2. Description of the Prior Art A cellular automobile telephone system offers its se-vice area divided into a plurality of small radio .15 zones. Each radio zone includes at least one radio base station which sets a communication channel between it and a mobile station. In such a cellular automobile telephone system, each radio zone is reduced in its size to enjoy Sless transmission power of radio equipment which corresponds to the size of the radio zone, and further repetitive use is made of signals of the same frequency for o oo effective use of frequencies in radio zones which are isolated from each other by distances where no deterioration coo• of communication quality is produced owing to interferences of signals of the same frequency.

Referring to FIG. 1 an outline of a cellular automobile telephone system of the type described dbove is illustrated.

A mobile body 1 performs speech communication through a radio station through a code transmission channel A. An input speech from a speech input terminal IN is encoded by a speech signal encoder device 11 in the mobile body 1 and transmitted to the radio station 2 from radio equipment 12.

The speech signal encoder device 11 includes a system known as being used in a North American cellular automobile telephone system using an algorithm, one version of a CELP (Code Excited Linear Predictive Coder) system, wherein a 64 kbps speech signal transmitted from an exchange is divided into 20 ms frames, and is converted into a 6.7 kbps speech signal by extracting the feature of the original transmitted speech signal for each frame.

The speech signal encoder device 11 fur-ther includes 0 an European cellular automobile telephone system as another speech signal encoder device using an algorithm of RPE-LTP (Regular Pulse Excited with Long Term Predictive Coder), wherein a speech signal is divided into 20 ms frames as in the aforementioned North American cellular system, and is converted to a 13 kbps speed signal by extracting the S. feature of the original speech signal for each frame. Such a speech signal possesses as its chief features a speech spectral envelope component, a pitch component, and an excitation signal, and transmission information is compressed by encoding each feature component with minimum information. The speech signal is regenerated such that since a transmission signal has been encoded, divided into the speech spectrum envelpe component, pitch component, and excitation signal as described above, the excitation signal is first decoded in the speech decoder device 13 in conformity with a predetermined decoding technique, and then the pitch component is regenerated by applying the transmission signal to a digital filter representative of the pitch component, and finally the spectral envelope component is regenerated by applying the transmission signal to a digital filter representative of the spectral envelope component.

oooo eeoe The encoded speech signal received by radio equipment 21 in the radio station is encoded by a speech signal decoder device 22 into the speech signal, and is oleo• transferred to the exchange 3.

In contrast, the speech signal from the exchange 3 is encoded by a speech signal encoder device 23 in the radio •coo Sstation 2, and is sent from radio equipment 21. The encoded speech signal sent to the mobile body 1 mediating the S: transmission channel A is received by the radio equipment 12 and is decoded by a speech signal decoder device 13 into a regenerated speech signal, and is taken out from a speech output terminal OUT.

In this situation, as the mobile body 1 .noves to another location, say, the location of a mobile body 1', the mobile body 1 is switched in its associated code transmission lines from a state where the mobile body 1 performs speech communication through the tasmission line A to a state where it performs the speech transmission through the code transimission channel B.

The radio station 2, which is now to switch the code tansmission channels mediating the signal from the mobile body 1 or 1i', monitors at all times the state of electric waves transmitted from the mobile body 1. The monitoringis generally done by observing electric field intensity, or may be done by utilizing other parameters, say, the rate of variations of fading.

With the state of the electric waves being deteriorated, the radio station 2 requires the exchange 3 o 0 to switch the present rnsmission channel A to another tansmission channel. The exchange 3, whch receives the request, instructs a radio station 4 to monitor the state eo'e of the radio waves from the mobile body 1 or As the 000 radio stattion 4 judges the state of the radio waves monitored thereby to be satisfactory compared with th( state of the radio waves monitored by ten radio station 2, the exchange 3 issues a switching control signal to switch the channels, to the mobile body 1 or 1' through the radio station 2.

The control signal is detected by a control section 14, whereby the radio equipment 12 is switched in its radio frequency to a channel corresponding to the radio station 4. The radio station which is with the same construction as that of the radio station 2, starts to send a silent signal to the exchange 3 a predetermined interval before a switching timing when the switching is performed, and once, after the switching, a speech code is received by the rad-io equipment 41, the speech code is decoded in a speech signal decoder device 42 and a regenerated speech signal is transmitted to the exchange 3.

The speech signal decoder device 22 in the radio ooeo Sstation 2, once the transmission channels have been S switched and the speech code is not transmitted thereto, interrupts its decoding without delay and issues a silent signal to the exchange 3.

The exchange 3 initially selects the speech signal from the radio station 2 and transfers it to the transmission channel output terminal OUT. Thereafter, it synthesizes the speech signal from the radio station 2 and S the speech signal from the radio station 4 in a certain interval around the time the channel switching is performed, and transmits the synthesised signal to the transmission channel output terminal OUT. Further, the exchange 3 selects the signal from the radio station 4 and transfers it to the transmission channel output terminal OUT.

The exchange 3 initially transfers a speech signal inputted through a transmission channel input terminal IN to the radio station 2 which required the switching of the transmission channels, but thereafter it transfers the speech signal to the radio station 4 for which switching is to be performed.

Such a prior art system has a difficulty that speech quality is deteriorated owing to instantaneous interruption of a speech signal because the switching of the code transmission channels is performed during a speech irrespective of the operation of a speech signal encoder/decoder device.

SUMMARY OF THE INVENTION To solve the difficulties with the prior )rt, it is an object of the present invention to provide a speech signal encoder/decoder device in mobile communication capable of ensuring a satisfactory speech signal even though code transmission channels are switched during communication.

S.Therefore, the invention discloses a speech signal Lncoder/decoder device, for use in mobile communications wherein switching of q transmission channel used for the communication of encoded speech information bets een a mobile communications unit and successive radio stations occurs as said mobile communications unit moves, said mobile communications unit comprising said sperch signal encoder/decoder device and •radio equipment, said speech signal encoder/decoder device comprising: i a speech signal encoder device for encoding an input speech signal in 25 conformity with a predetermined encoding procedure, said speech signal encoder device including initialising means for initialising a memory circuit for speech signal encoding in response to said switching; and a speech signal decoder device for decoding an encoded speech signal in conformity with a predetermined decoding procedure.

The invention further discloses a speech signal encoder/decoder device, for use in mobile communications wherein switching of a transmission channel used for the communication of encoded speech information between a mobile communications unit and successive radio stations occurs as said mobile communications unit moves, said mobile communica' ons unit comprising said speech signal encoder/decoder device and radio equipmext.

InliboIOO291 rlhk said speech signal encoder/decoder device comprising: a speech signal encoder device for encoding an input speech signal in conformity with a predetermined encoding procedure; and a speech signal decoder device for decoding an encoded speech signal in conformity with a predetermined decoding procedure, said speech signal decoder device including initialising means for initialising a memory circuit for speech signal decoding in response to said switching, and ex:rapolation means for extrapolating the speech signal using the past received codes in response to said switching of said transmission channels.

The invention yet further discloses a speech signal encoder/decoder device, for use in mobile communications wherein switching of a transmission channel used for the communication of encoded speech information between a mobile communications unit and successive radio stations occurs as said mobile communications unit moves, said mobile communications unit and said radio stations comprising said speech signal encoder/decoder device and radio equipment, said speech signal encoder/decoder device comprising: a speech signal encoder device for encoding an input speech signal in conformity with a predetermined encoding procedure, said speech signal encoder device including initialising means for initialising a memory circuit for speech signal encoding 20 in response io said switching; and "a speech signal decoder device for decoding an encoded speech signal in S"conformity with a predetermined decoding procedure.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram illustrating in an exemplified manner a prior art S 25 speech signal encoder/decoder device in mobile communication; FIG. 2 is a block diagram illustrating an embodiment of a speech signal :0"•encoder/decoder device in mobile BA41 i'r C in AhbeoJ0291 (Ilk -8comnunication according to the present invention.

FIG. 3 is a block diagram illustrating a speech signal encoder device in the speech signal encoder/decoder device of FIG. 2; FIG. 4 is a block diagram illustrating a speech signal decoder device of the speech signal encoder/decoder device of FIG. 2; FIG. 5 is a timing chart illustrating timings of initialization and extrapolation in the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS In the following, a principle of the present invention will be described prior to the description of a preferred embodiment of the present invention.

speech signal decoded by a speech signal decoder device not only determines a regenevated speech signal only ,io depending upon an encoded signal received presently but also it reflects an influence of past received codes. The speech signal encoded by a speech signal encoder device of S• a mobile body is decoded through different speech signal decoder devices before and after the switching of transmission channels. Thereupon, the speech signal decoder device to be switched in its communication channel fails to reflect the influence of past received codes and 1'ence *ooa brings about discontinuity of the speech signal.

Accordingly, in the case where channel switching is performed and a code to be sent to a new speech signal encoder device is estimated, the speech encoder device of a mobile body is initialized to eliminate the influence of past speech codes, and thereafter encoded.

Further, since the speech signal decoder device of a mobile body decodes a speech code encoded by a different speech signal encoder device, the speech signal decoder device fails to correctly reflect the influence of past received codes owing to the switching of the channel. In order to generate intentionally the influence of the past speech encoding after the transmission channel is switched, the present invention is adapted to extrapolate a speech signal using past received codes with the aid of an S extrapolation circuit. The present invention further ':15 improves the quality of the speech signal in view of the sense of hearing by weighted synthesis of an extrapolated ooe signal by the extrapolation circuit and a regenerated signal. In what follows, a preferred embodiment of the .present invention will be described with reference to the accompanying drawings.

Referring to FIG. 2, there is illustrated an embodiment of a speech signal encoder/decoder device according to the present invention in the form of a block diagram.

A basic circuit construction of the speech signal encoder/decoder device of the present invention is identical to the prior art speech signal encoder/decoder device of FIG. i, but is different in circuit constructions of speech signal encoder devices and speech signal decoder devices in a mobile body and a radio station.

Referring to FIG. 3, there is ilustrated in a block diagram an embodiment of the speech signal encoder device lla, 23a, 43a of FIG. 2. A speech signal decoder circuit receives a discrete speech signal from an input terminal 51 e 10 and encodes the same with use of a predetermined encoding procedure and transmits a resulting speech code to an output terminal 52. The speech signal encoder circuit 5 is constructed in the same manner as in the prior art encoder S device 11 illustrated in FIG. i.

:15 A control circuit 6 receives a code transmission channel switching control signal from a control input e ee g **terminal 53, and issues an initialization control signal to ooo• an initialization circuit 7 in a timing when the switching of the code transmission channel is performed. The 20 intilialization circuit 7, once receiving the intialization control signal, initializes a predetermined memory circuit provided in the speech signal encoder circuit The foregoing speech signal encoder devices used in the North American and European cellular automobile telephone system are adapted to process a speech signal for -11each predetermined time interval frame and send a resulting encoded result. The coil is specified not only by an input speech signal in a present time frame but also by a processed result of past frames. Contents in a memory circuit, which reflects a processed result of past frames, are therefore initialized.

Such memory circuits include those for short-term predictive filters and short-term analytic filters and long-term analytic filters, and those for storing previous o:.e :10 frame codes, and so on. Contents of these memories are Sinitialized. Referring to FIG. 4, there is illustrated in V a block diagram an embodiment of the speech signal decoder device 13a, 22a, 42a illustrated FIG. 2. A speech signal decoder circuit 8 decodes the received code inputted through a reception terminal 81 with use of a predetermined decoding process, and transmits a regenerated speech signal go to a weighting synthesis circuit 20. The speech signal decoder circuit 8 is of the same construction as that of the prior art speech signal decoder circuit 13 illustrated 0 in FIG. 1.

A control circuit 30 receives a code transmission channel switching control signal from a control input terminal 83 and issues an extrapolation control signal to an extrapolation circuit 9 in a predetermined interval when the code transmission channel is switched. The extrapolation -12circuit 9 estimates an extrapolated speech signal in an interval when tb extrapolation control signal is inputted, on the basis of pest rf ived codes.

For such extrapolation of a speech signal, varieties of techniques are widely known and useable. There is for example available an extrapolation technique disclosed in "Lost Frame Substitution and Muting (GSM Full Rat- Speech Transcoding)" (CEPT/CCH/GSM Recommendation 06, 11.5 July 1989).

:10 In the extrapolation technique disclosed ir. the above S. reference, a pitch component enjoys a previous frame code as it is, and an excitation signal employs itself but

S

including fine variations added thereto in conformity with a predetermined technique. Hereby, a speech signal is regtnerated. This originates from a knowledge that feature parameters of a speech signa. such as a spectral envelope and a pitch component among feature parameters of the same are less varied in such a short-term interval as across an interface between adjacent frames. Use, however, of quite the same set of the speech regeneration parameters causes a problem of a regenerated speech being mechanical, and hence the excitation signal includes randomness of a certain kind added thereto.

The control circuit 30 issues an initialization control signal to the initialization circuit 10 in a -13timing of the code transmission channel being switched. The initialization circuit 10, once receiving the initialization control signal, initializes a predetremined memory circuit disposed in the speech signal decoder circuit 8.

In the foregoing speech signal decoder device used in the North American telephone systems, a speech is processed for each predetermined length frame to in'.tialize contents in a memory in which decoding data concerning past frames have been stored.

A weighting synthesis circuit 20 transfers the regenerated speech signal from the decoder circuit 8 intactly to the output termnal 82 with no extrapolation control signal being issued, and weight-synthesizes the extrapolation signal from the exterpolation circuit 9 and the regenerated speech signal from the decoder circuit 8 and outputs a resulting weighted, synthesized signal to the output terminal 82 in the time interval the extrapolation coiitrol signal is issued.

It is now assumed that the regenerated speech signal from the* decoder circuit 8 is the extrapolation signal from the extrapolation circuit 9 is and the output signal from the weighting synthesis circuit 20 is These signals z(n) are discrete, and with n and T each assumed to be integers and a sampling period, present time t is expressed by a relation t=nT.

-14- The weighting-synthesis circuit 20 outputs intactly the regenerated signal x(n) from the decoder circuit 8, which signal x(n) satisfies an equation when no extrapolation control signal is issued from the control circuit The weighting-synthesis circuit 20 takes x(n) and y(n) as inputs thereto once the extrapolation control signal is issued, and outputs the following function: 10 where the interpolation control signal is assumed to be S inputted to the weighting synthesis circuit 20 when n=n0 is satisfied. Herein, f(n) is a predetermined function, say, f(n)=n/N where n is a constant indicative of the time interval of the weighting synthesis being done.

Referring to FIG. 5, there is illustrated a timing chart showing initializations and extrapolation of the encoder device and the decoder device in the mobile body 1 and radio stations 2 and 4 in the above embodiment.

The exchange 3 simultaneously transmits a switching control signal to the speech signal encoder device and the decoder device of the mobile body 1 and the radio stations 2 and 4. The mobile body 1, as receiving the switching signal, initializes the speech signal encoder device while initializing and extrapolating the speech signal decoder device. The speech signal decoder device 22 of the radio station 2 may be initialized and extrapolated. The speech signal encoder device 43 of the radio station 4 may be initialized. The speech signal decoder device 42 of the radio station 4 may be initialized.

It is herein noticed that in the above embodiment the extrapolation circuit 9 obtained the speech extrapolation sco 1 i0 signal by directly estimating past received codes but in anothex speech extrapolation technique such an extrapolation .signal is estimable using an intermediate result yielded in the course of speech decoding or the regenerated speech signal so that data required for the extrapolation may be transmitted from the speech signal decoder circuit 8 to the extrapolation circuit 9.

According to the present inventici, as described above, the influence of past speech codes is eliminated even though the code transmission channel is switched in the middle of a speech where the mobile body is in the course of speech encoding so that quality of the speech signal is prevented from being deteriorated.

Further, the influence of the past speech code is generated intently even though the code transmission channel is switched in the middle of a speech where the -16mobile body is in the course of speech decoding, so that a speech signal is prevented from being deteriorated in its quality.

Although in the above embodiment, all speech signal encoder devices lla, 23a and 43a of the mobile body 1 and the radio stations 2 and 4 were! initialized and all speech signal decoder devices 13a, 22a and 42a were initialized and extrapolated, in the present invention only the mobile body 1 or only the radio stations 2 and 4 may be intialized or 10 extrapolated. Further, the transmission channel is switchable without deteriorating the quality of a speech signal by performing initialization and extrapolation at least to one of the speech of the speech signal encoder device or the speech signal encoder device.

Claims

1. A speech signal encoder/decoder device, for use in mobile communications wherein switching of a transmission channel used for the communication of encoded speech information between a mobile communications unit and successive radio stations occurs as said mobile communications unit moves, said mobile communications unit comprising said speech signal encoder/decoder device and radio equipment, said speech signal encoder/decoder device comprising: a speech signal encoder device for encoding an input speech signal in conformity with a predetermined encoding procedure, said speech signal encoder device including initialising means for initialising a memory circuit for speech signal encoding in response to said switching; and a speech signal decoder device for decoding an encoded speech signal in conformity with a predetermined decoding procedure.

2. A speech signal encoder/decoder device according to claim I, wherein said speech signal decoder device includes initialising means for initialising a memory S-circuit for speech signal decoding in response to said switching, and extrapolation means for extrapolating a speech signal using past received codes in response to said switching.

3. A speech signal encoder/decoder device according to claim 2, wherein said speech signal decoder device includes a decoder circuit for speech decoding, and synthesis means for weighting an output from the decoder circuit and an extrapolation signal from the extrapolation means with a predetermined weighting factor and synthesising a resulting weighted signal. 25

4. A speech signal encoder/decoder device, for use in mobile communications wherein switching of a transmission channel used for the cormnunication of encoded speech information between a mobile communications unit and successive radio stations occurs as said mobile communications unit moves, said mobile communications unit comprising said speech signal encoder/decoder device and radio equipment, said speech signal encoder/decoder device comprising: a speech signal encoder device for encoding an input speech signal in conformity with a predetermined encoding proceduie; and a speech signal decoder levice for decoding an encoded speech signal in conformity with a predetermined decoding procedure, said speech signal decoder device )cj RA1 including initialising means for initialising a memory circuit for speech signal decoding S in response to said switching, and extrapolation means for extrapolating the speech (n:\lbe100291 :rhk -18- signal using the past received codes in response to said switching of said transmission channels.

A speech signal encoder/decoder device, for use in mobile communications wherein switching of a transmission channel used for the communication of encoded speech information between a mobile communications unit and successive radio stations occurs as said mobile communications unit moves, said mobile communications unit and said radio stations comprising said speech signal encoder/decoder device and radio equipment, said speech signal encoder/decoder device comprising: a speech signal encoder device for encoding an input speech signal in conformity with a predetermined encoding procedure, said speech signal encoder device including initialising means for initialising a memory circuit for speech signal encoding in response to said switching; and a speech signal decoder device for decoding an encoded speech signal in conformity with a predetermined decoding procedure.

6. A speech signal encoder/decoder device according to claim 5 wherein said speech signal decoder device includes initialisation means for initialising tize memory circuit for speech signal decoding in response to said switching, and extrapolation means for extrapolating a speech signal using past received codes in response to said switching.

7. A speech signal encoder/decoder device according to claim 6 wherein said speech signal decoder device includes a decoder circuit for speech signal decoding, and synthesis means for weighting an output from said decoder circuit and an extrapolation signal from said extrapolation means with a predetermined weightirng S: 25 factor for synthesis of said speech signal.

8. A speech signal encoder/decoder device substantially as hereinbefore described with reference to Figure 2 to Figure 5 of the accompanying drawings. DATED this Third Day of May 1995 NEC Corporation Patent Attorneys for the Applicant SPRUSON FERGUSON In!\llb100291I :rhk Speech Signal Encoder/Decoder Device in Mobile Communication Abstract of the Disclosure In mobile communication, a satisfactory speech signal is transmitted without discontinuity even when code transmission channels are switched during operation. A speech signal encoder device (lla,23a,43a) in both mobile stations and radio stations (2,4) comprises a speech signal encoder circuit an initialization circuit and a control circuit The control circuit issues an initialization to an initialization circuit timed for when switching of the code transmission channel (53) takes place. The initialization circuit initializes the contents in a memory in a speech signal encoder circuit Since the memory contents in the encoder circuit are initialized with the switching timing of the transmission channels in such a manner, the influence of the previous speech codes is eliminated and hence discontinuity eliminated. Figure 3. o ft ft r• 5455M/GMM